WASHINGTON -- An envelope laced with the lethal poison Ricin and addressed to a U.S. senator was found at Maryland mail processing facility, officials said Tuesday.

What is ricin?

Here's a look at ricin, a poison made from waste remaining after castor beans are processed into oil.

How it works:

Ricin gets inside cells and prevents them from making proteins. Without proteins, cells die. Forms: Powder, mist or pellet. Methods of exposure: Ingestion, inhalation or injection. Symptoms: If inhaled, respiratory distress, fever, cough, nausea, sweating, fluid buildup in lungs. If ingested, vomiting and bloody diarrhea, seizures, hallucinations. Symptom onset: If inhaled, possibly within eight hours. If ingested, typically less than six hours. Results: Low blood pressure and respiratory failure; spleen, liver and kidney malfunction, leading to death. Time of death: Could be 36 to 72 hours, depending on the dose and method of transmission. Treatment: No antidote or specific vaccine, but effects can be treated.

Source: Centers for Disease Control and Prevention.

Los Angeles Times

The envelope, intended for the office of Sen. Roger Wicker (R-Miss.), was discovered at a location that screens letters bound for congressional offices in Washington, according to a law enforcement official. More tests are being conducted to confirm the substance.

[Updated at 5:47 p.m.: Late Tuesday, Wicker issued a statement thanking “our law enforcement officials for their hard work and diligence in keeping those of us who work in the Capitol complex safe. Gayle and I appreciate everyone’s thoughts and prayers.”]

Senators were told of the situation late Tuesday during a classified briefing to discuss security matters, primarily the Boston Marathon bombing, with Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller. The Senate sergeant at arms was also present.

Senate post offices were closed and will undergo regular decontamination procedures, according senators at the briefing.

A memo from the House sergeant at arms said that House post office employees “should be vigilant in their mail-handling processes,” but that no suspicious mail had been received there.

Ricin is a poison that can be deadly in small amounts, according to the Center for Disease Control. It can be inhaled or ingested, and there is no antidote.

Mailings to the Capitol complex undergo intensive security screenings that were enhanced after letters containing anthrax were sent to congressional offices following the Sept. 11, 2001, terrorist attacks.

The suspicious envelope tested positive for Ricin after an initial screening, according to a law enforcement official. It is unclear when the envelope was first intercepted.

The FBI's Baltimore field office is working with the Capitol Police to investigate the source of the envelope.

“The bottom line is the process we have in place worked,” Sen. Claire McCaskill (D-Mo.) said as she emerged from the briefing.

She suggested that the sender was known to authorities. “Evidently this person -- this person who is a suspect -- writes to a lot of members,” she said.

Terrance W. Gainer, the Senate sergeant at arms, said in a letter to Senate staffers that the envelope was not "outwardly suspicious."

He said it had no return address and was postmarked from Memphis, Tenn.

“While we have no indication that there are other suspect mailings, it is imperative to follow all mail handling protocols,” Gainer's letter said.

For lawmakers’ state offices, Gainer suggested senators use the “Postal Sentry” system – a desktop device that provides airflow and filtration to reduce the threat from harmful agents while opening mail.

[Updated at 5 p.m.: The original post was updated to add information from Gainer's letter to Senate staff members.]

The off-site facility is expected to be closed for two or three days as the investigation continues, officials said.

“We will have to notify our state offices, which don't have the screening facilities to look out for it,” said Sen. Angus King (I-Maine).

"Obviously it's concerning,” said Sen. Jeff Flake (R-Ariz.) “It rarely gets to a member before it goes through a lot of staff. That's a big concern, obviously, for all of us. We're very anxious to get more details."